F. Smole

Effect of surface roughness of ZnO:Al films on light scattering in hydrogenated amorphous silicon solar cells

Abstract Experimental investigation combined with computer modeling is used for analysis of light scattering process in hydrogenated amorphous silicon (a-Si:H) solar cells deposited on textured glass/ZnO:Al substrates. Descriptive scattering parameters—haze and angular distribution functions (ADFs)—for the textured ZnO:Al films with different surface roughness are determined. The haze parameters of all internal interfaces in the a-Si:H solar cells are calculated using equations of scalar scattering theory calibrated on the measurements of the substrates. The ADFs determined for the substrates are modified and applied to the internal interfaces. The scattering parameters are incorporated in our optical model and used to simulate the effect of the ZnO:Al surface roughness on the quantum efficiency (QE) of the solar cells. The simulations reproduce the measured QE of all solar cells with different roughness of the substrate very well.

Infrared light management in high-efficiency silicon heterojunction and rear-passivated solar cells

Silicon heterojunction solar cells have record-high open-circuit voltages but suffer from reduced short-circuit currents due in large part to parasitic absorption in the amorphous silicon, transparent conductive oxide (TCO), and metal layers. We previously identified and quantified visible and ultraviolet parasitic absorption in heterojunctions; here, we extend the analysis to infrared light in heterojunction solar cells with efficiencies exceeding 20%. An extensive experimental investigation of the TCO layers indicates that the rear layer serves as a crucial electrical contact between amorphous silicon and the metal reflector. If very transparent and at least 150 nm thick, the rear TCO layer also maximizes infrared response. An optical model that combines a ray-tracing algorithm and a thin-film simulator reveals why: parallel-polarized light arriving at the rear surface at oblique incidence excites surface plasmons in the metal reflector, and this parasitic absorption in the metal can exceed the absorption in the TCO layer itself. Thick TCO layers—or dielectric layers, in rear-passivated diffused-junction silicon solar cells—reduce the penetration of the evanescent waves to the metal, thereby increasing internal reflectance at the rear surface. With an optimized rear TCO layer, the front TCO dominates the infrared losses in heterojunction solar cells. As its thickness and carrier density are constrained by anti-reflection and lateral conduction requirements, the front TCO can be improved only by increasing its electron mobility. Cell results attest to the power of TCO optimization: With a high-mobility front TCO and a 150-nm-thick, highly transparent rear ITO layer, we recently reported a 4-cm2 silicon heterojunction solar cell with an active-area short-circuit current density of nearly 39 mA/cm2 and a certified efficiency of over 22%.

Optical modeling of a-Si:H solar cells deposited on textured glass/SnO2 substrates

In this article we determine descriptive scattering parameters—haze and angular distribution functions—of scattered light for textured glass/SnO2 Asahi U-type substrates. These scattering parameters are input parameters of our optical model that enables us to analyze multilayer optical systems with rough interfaces. The scalar scattering theory is used to calculate the haze parameters of all internal rough interfaces in the a-Si:H solar cells deposited on the glass/SnO2 substrates. In the equations of the scalar scattering theory the correction functions are introduced in order to match the calculations with the measurements of the haze parameters of the substrates. The angular distribution functions of the substrates are applied to the rough internal interfaces. Using these scattering parameters we investigate the optical behavior of a-Si:H solar cells with different intrinsic layer thicknesses deposited on the textured glass/SnO2 substrates with different roughnesses.

Amorphous silicon oxide window layers for high-efficiency silicon heterojunction solar cells

In amorphous/crystalline silicon heterojunction solar cells, optical losses can be mitigated by replacing the amorphous silicon films by wider bandgap amorphous silicon oxide layers. In this article, we use stacks of intrinsic amorphous silicon and amorphous silicon oxide as front intrinsic buffer layers and show that this increases the short-circuit current density by up to 0.43 mA/cm2 due to less reflection and a higher transparency at short wavelengths. Additionally, high open-circuit voltages can be maintained, thanks to good interface passivation. However, we find that the gain in current is more than offset by losses in fill factor. Aided by device simulations, we link these losses to impeded carrier collection fundamentally caused by the increased valence band offset at the amorphous/crystalline interface. Despite this, carrier extraction can be improved by raising the temperature; we find that cells with amorphous silicon oxide window layers show an even lower temperature coefficient than referenc...

Band‐gap engineering in CdS/Cu(In,Ga)Se2 solar cells

Using a numerical device simulation program, the band‐gap engineering in CdS/Cu(In,Ga)Se2 solar cells is examined. The device physics of different design concepts is analysed. Normal band‐gap grading improves performance, especially due to the additional quasi‐electric field, and the analysis showed that the best results are achieved if the grading extends from the highest band‐gap value at the back up to the space charge region. The double grading concept does not yield further improvement, because the front grading—even if located in the space charge region—repels the minority carriers (electrons) away from the CdS interface, and consequently, the fill factor drops significantly. Notch structures in the base also exhibit lower performance than the uniform band‐gap base due to the lower open‐circuit voltage and poorer fill factor. Therefore, the best results are achieved by a normal grading in a Cu(In,Ga)Se2 base from the edge of the space charge region to the back contact.

Analysis of TCO/p(a-Si:C:H) heterojunction and its influence on p-i-n a-Si:H solar cell performance

The ASPIN computer simulator, which enables analysis of transparent conducting oxide (TCO)/a-Si:C:H/a-Si:H/TCO heterostructures, was used to examine the influence of different front TCO/p(a-Si:C:H) heterojunctions on TCO/p-i-n/TCO/metal a-Si:H solar cell performance. Separate analysis of TCO/p(a-Si:C:H) structure for both SnO2 and ZnO indicates that the mismatch between the high contact potential and the measured potential barrier at the p-layer surface can be resolved by a large density of interface defect states, causing a steep potential decrease in the interface. Analysis of the detrimental effects of a-Si:C:H chemical oxidation in SnO2/p(a-Si:C:H), which were simulated by the increased surface state density in the a-Si:C:H, showed that the potential barrier in a p-layer with oxidized surface is increased. The impact of both TCO/p(a-Si:C:H) interface states and a-Si:C:H surface states on the photoelectric properties of p-i-n a-Si:H solar cells is discussed, and a possible improvement of Voc is envisaged.

Examination of blocking current-voltage behaviour through defect chalcopyrite layer in ZnO/CdS/Cu(In,Ga)Se2/Mo solar cell

Abstract Blocking current-voltage behaviour of ZnO/CdS/Cu(In,Ga)Se2/Mo solar cells, which is either temperature- or light-conditioned, is examined using a comprehensive numerical device simulator. Effects of defect states in the defect-chalcopyrite layer and at the CdS/defect-chalcopyrite interface are investigated. Acceptor-like defect states either in a defect-chalcopyrite layer or at the CdS/defect-chalcopyrite interface cause different trapping under red light or white light. This results in different potential profiles throughout the structure, which determine the changeable I−V behaviour under forward bias. Simulation results show that these acceptor-like defect states can also control the temperature-conditioned blocking I−V behaviour.

Analysis of lateral transport through the inversion layer in amorphous silicon/crystalline silicon heterojunction solar cells

In amorphous/crystalline silicon heterojunction solar cells, an inversion layer is present at the front interface. By combining numerical simulations and experiments, we examine the contribution of the inversion layer to lateral transport and assess whether this layer can be exploited to replace the front transparent conductive oxide (TCO) in devices. For this, heterojunction solar cells of different areas (2 × 2, 4 × 4, and 6 × 6 mm2) with and without TCO layers on the front side were prepared. Laser-beam-induced current measurements are compared with simulation results from the ASPIN2 semiconductor simulator. Current collection is constant across millimeter distances for cells with TCO; however, carriers traveling more than a few hundred microns in cells without TCO recombine before they can be collected. Simulations show that increasing the valence band offset increases the concentration of holes under the surface of n-type crystalline silicon, which increases the conductivity of the inversion layer. U...

Effects of abrupt and graded a‐Si:C:H/a‐Si:H interface on internal properties and external characteristics of p‐i‐n a‐Si:H solar cells

Using a computer simulation, the effects of abrupt and graded a‐Si:C:H/a‐Si:H interfaces on the performance of a‐Si:H p‐i‐n solar cells are discussed. It is shown that structures with graded heterojunction transitions possess much lower recombination near the junction and a higher accelerating built‐in electric field in the i layer, both of which increase the open‐circuit voltage and improve the solar cell fill factor.

Optimization of a-Si:H-based three-terminal three-color detectors

Three-terminal three-color n-SiC:H/a-Si:H-based TCO/PINIP/TCO/PIN/metal detectors are presented. Assemblies having different surface roughness of transparent conducting oxide (TCO) layers are compared with regard to the external steady-state characteristics and transient behavior. The roughness of the sputtered TCO surface can be modified by an etching treatment. With the selection of smooth or textured TCO surfaces, the wave propagation of light within the device is controlled. This design technique can be exploited to optimize the color separation and improve the reproducibility of spectral responsivities in the assemblies. The examined assemblies exhibit very selective spectral responsivity for the fundamental chromatic components (red-green-blue) and a linear photocurrent-generation rate relationship over more than five orders of magnitude of illumination intensity. Since the color detection of blue and green light is performed in the PINIP structure by bias switching, the transient current response of the PINIP structures is investigated. A reciprocal relationship between the delay time and illumination intensity is established. An optimum operation region for the switching voltages is determined with regard to the quality of color separation, dynamic range, and transient behavior.